Until now, wet type nonwoven fabrics have largely been used for the application of various types of filters. Regarding raw materials for wet type nonwoven fabrics, polyester fiber, micro glass, polyethylene synthetic pulp, and para-aramid synthetic pulp are known (see, for example, Patent Documents 1 and 2).
However, when a conventional wet type nonwoven fabric including a polyester fiber is used as a filter, since the pore diameter of pores appearing on the surface of the wet type nonwoven fabric is nonuniform, there is such problem that a sufficient collection efficiency cannot be obtained. On the other hand, when a wet type nonwoven fabric including micro glass or synthetic pulp is used as a filter, since the fiber length and fiber diameter of the micro glass or synthetic pulp is nonuniform, there is such problem that the quality of the filter is insufficient.
Meanwhile, recently, the research and development have energetically been performed for fibers having a small fineness (see, for example, Patent Documents 3-5).
Patent Document 1: JP-A-2006-241654
Patent Document 2: Japanese Patent No. 3678511
Patent Document 3: JP-A-2004-162244
Patent Document 4: WO 2005/095686
Patent Document 5: WO 2005/080679